The historical development and use of paper as a means to record human events is extremely rich and illustrious. However, most of the technological advances associated with papermaking have evolved slowly, or have sprouted only after long time periods have passed. For instance, until 1803, nearly two-hundred and fifty years after the development of the printing press, hand paper production methods developed by the Chinese over two-thousand years ago remained the primary means by which paper was produced. In 1803 mechanical methods to produce paper were developed. Although these newer mechanical methods were themselves viewed as a boon to the paper production industry, they actually exacerbated paper shortage problems because they increased the demand for vegetative fibers, the essential ingredient used in the production of paper.
In 1843, Friedrich Gottleib Keller addressed the vegetative fiber shortage problem by devising a method and apparatus for producing and manufacturing wood pulp using a stone grinding wheel known as a “pulpstone”. Keller's invention was timely because the Industrial Revolution was just entering full swing and demand for paper fiber was high. As a result, pulp production facilities having huge pulpstones were constructed for continuously grinding massive amounts of wood into pulp. Today, Keller's pulpstone grinding methods remain relatively unchanged, except for increases in capacity and efficiency due to refinements in grinder design. In fact, pulpstones are still used to produce large amounts of the wood pulp used in paper products today. FIG. 1 shows a typical wood pulp grinder 10 comprising a rotating pulpstone 12 and a plurality of pushers 14 for pressing logs 9 against an outer grinding surface 12A of the pulpstone. As seen in FIG. 1, pulpstone 12 rotates in a counterclockwise direction and logs 9 are arranged such that their longitudinal axes are parallel to the axis of rotation of the pulpstone. As pulpstone 12 rotates, logs 9 are fed under pressure by pushers 14 into engagement with pulpstone surface 12A to produce wood pulp.
As can be seen in FIG. 2, pulpstone 12 generally includes abrasive grains 15 held together by a bonding material 16, and are typically manufactured from either ceramic or cement bonded abrasive. Pulpstone surface 12A is shown in detail as including alternating land areas 17 and groove areas 18. As pulpstone 12 rotates and the timber 9 is pushed against pulpstone surface 12A, land areas 17 come into contact with the timber and groove areas 18 pass over the surface of the timber, thereby creating oscillation between mechanical compression and decompression that generates heat. The heat softens the lignin of the wood and the rotational forces acting on the timber loosen and remove the wood fiber. Because of the extreme pressures, high frictional forces, and heat generated during the grinding of the timber, land areas 17 of pulpstone surface 12A eventually begin to wear and widen, and the abrasive grains 15 begin to dull. The extent of such surface wear often varies over the axial length of pulpstone 12. Consequently, more and more energy must be expended in order to maintain a consistent quality and output of pulp. Thus, the surface quality and groove pattern of the pulpstone play a very critical role in efficient production of the desired quality pulp. It is, therefore, extremely desirable to ensure that the land/groove pattern on the pulpstone surface is maintained by regular “sharpening” or “dressing” of the pulpstone surface.
The term “pulpstone sharpening” is a misnomer; pulpstone sharpening does not actually sharpen the abrasive of a pulpstone. Rather, pulpstone sharpening fractures the softer bonding material of the pulpstone to remove the dull, older abrasive grains and to uncover the sharper, newer grains and to maintain the desired grooved pattern.
Referring to FIG. 3 of the drawings, a known procedure for sharpening a pulpstone is illustrated. To sharpen a pulpstone, a sharpening burr 20 is journaled in a forked end 19 of a cross-slide (not shown), which in turn mounted on a traversing carriage of a lathe (also not shown) such that the burr's axis of rotation is parallel to that of the pulpstone. As seen in FIG. 4, burr 20 has a plurality of spaced apart teeth 22 on its outer peripheral surface that can be forced into pulpstone surface 12A to a predetermined depth by adjustment of the cross-slide mechanism. Once burr 20 has been aligned at a side edge of pulpstone 12 and the burr depth has been set, the pulpstone is rotated, thereby imparting rotational motion to the sharpening burr. Burr 20 is then caused to traverse linearly across pulpstone surface 12A as indicated by the bi-directional arrows in FIG. 3. The traversal of sharpening burr 20 under pressure forms a pattern in pulpstone surface 12A by removing the old, abrasive grains and uncovering the sharp, new abrasive grains. The traversal process is repeated several times with a new sharpening burr as necessary to produce the desired pulpstone surface pattern. During the pulpstone sharpening process, the penetration depth of sharpening burr teeth 22 is critical. Proper tooth penetration depth fractures and removes abrasive grains on the surface of a pulpstone, however penetration that is too deep tends to fracture the deeper pulpstone surface bonds, creating wide grooves that promote pulpstone surface instability. Wide pulpstone grooves are undesirable because they produce long vegetative fibers and fiber bundles that are associated with lower grade pulps. In addition, pulpstone surface instability is also viewed as undesirable since it typically causes the premature wear of the pulpstone surface and failure of the groove pattern. Thus, pulpstone surface pattern and groove depth are viewed as important factors for manufacturing consistent and high quality pulps.
Pulpstone sharpening also serves several additional useful purposes. First, sharpening reduces the grinding surface area of a pulpstone, lowering energy consumption. Second, sharpening controls the oscillatory frequency of compression and decompression of the wood fibers, thus controlling the requisite heat that is used to release wood fibers from the lignin. Third, pulpstone sharpening cleans the pulpstone pores and prevents grinding surface overheating, cracking, and premature wear by carrying water to the grinding zone for cooling and lubricating purposes. Finally, pulpstone sharpening and groove pattern help to carry pulp out of the grinding zone.
Pulpstone sharpening burrs, for example burr 20 in FIGS. 3 and 4, have undergone very few changes over the past century, and there are currently four basic types in use today: spiral, diamond, fluted, and threaded. Spiral burrs have teeth that run parallel with one another and at an angle (known as the “lead angle”) to the rotational axis of the burr. When applied to the surface of a pulpstone, the spiral burr produces a series of diagonal impressions across the surface of the pulpstone. During grinding, the diagonal pattern formed on the pulpstone with a spiral burr removes wood fibers using a semi-shearing action. Diamond toothed burrs have teeth that form a pyramid or diamond shape. They are used primarily for truing pulpstones and for removing patterns on a pulpstone. Fluted burrs, such as burr 20 shown in FIG. 4, have teeth that run parallel to the rotational axis of the burr. When applied to a pulpstone surface, the fluted burr produces a pattern that is parallel to the rotational axis of the pulpstone. During grinding, the fluted pattern of the pulpstone removes long coarse fibers from the wood. A threaded burr produces a series of rings around the face of a pulpstone that produce a very light shearing action resulting in the production of a very high quality pulp. Pulpstone sharpening burrs are typically machined from a steel shell that is then heat treated for hardness and stress relief.
The teeth of a pulpstone sharpening burr play perhaps the most critical role in the production of wood pulp because they actually create the patterns that have such profound effects upon the amount, quality, and consistency of the final wood pulp product. Burr teeth function by fracturing pulpstone bond posts between abrasive grains, thereby forming the abrasive pattern on the surface of a pulpstone. Current and past tooth configurations have traditionally been exclusively “pointed-triangular”, as shown in FIG. 5. By “pointed-triangular” it is meant that the burr tooth 22 is formed by two converging opposite planar sides 23A and 23B that intersect at a substantially pointed tip 24. The included angle of tip 24 formed by the intersection of sides 23A and 23B is acute, ranging from 20° to 70°. Pointed-triangular tooth configurations suffer from two main drawbacks. First, because they are extremely sharp and pointed, they tend to wear unevenly when traversed across a pulpstone surface. Thus, pointed triangular teeth ultimately place an uneven pattern on the surface of the pulpstone, a result that is particularly undesirable because an uneven pulpstone pattern produces inconsistent grades of pulp depending upon axial position along the surface of the pulpstone. Second, pointed-triangular teeth can cause “deep bond breakage” in the surface of the pulpstone. Deep bond breakage is undesirable because it causes pulpstone surface and groove pattern instability that tends to promote the premature wear of the pulpstone. Because of deep bond breakage, a pulpstone must be taken offline and re-sharpened more frequently, resulting in more frequent equipment downtimes and lowered production outputs.
Thus, developing a sharpening burr that reduces the incidence of deep bond breakage and withstands wear such that a homogenous pattern can be placed across the surface of a pulpstone would be extremely beneficial to the pulpwood processing industry in terms of increased pulp quality and increased pulp production.